Low-pressure fixation of valvular tissue intended for implantation

ABSTRACT

A method of obtaining fixation of an animal aortic valve intended for implantation, including subjecting the aortic valve to a tanning fluid at a differential pressure across the valve leaflets in a direction to urge the valve leaflets closed and with the tanning fluid tending to distort the aortic valve. The differential pressure across the valve leaflets is greater than zero and less than about four mm Hg. An insert restrains the valve against substantial distortion during the time the valve is subjected to the tanning fluid.

This application is a division of the application Ser. No. 275,749 filedJune 22, 1981 now U.S. Pat. No. 4,372,743.

BACKGROUND OF THE INVENTION

Animal heart valves, such as porcine heart valves, have been used forimplantation in humans. To prepare the animal heart valve forimplantation, it is common practice to tan the valve. Tanning rendersthe animal tissue relatively inert with respect to the living hostenvironment and brings about fixation of the tissue, i.e., stabilizationof the tissue so that it has a fixed configuration.

As disclosed in Hancock et al U.S. Pat. Nos. 3,966,401 and 4,050,893 andAngell et al U.S. Pat. No. 3,983,581, it is known to tan animal heartvalves by utilizing a tanning fluid under pressures which may range from20 mm Hg to 120 mm Hg. These methods provide a substantial internalpressure on the heart valve. According to Hancock et al U.S. Pat. No.4,050,893, the use of pressures in the physiologic range, i.e., 80-120mm Hg causes the tissue to assume substantially its naturalconfiguration during the tanning process and in fixing the valve in itsnatural contour so that it will retain its shape following the tanningprocess.

Recent reports by Broom, Neil D., An In Vitro Study of MechanicalFatigue in Glutaraldehyde-Treated Porcine Aortic Valve Tissue,Biomaterials, Volume 1, Jan. 3, 1980, and Broom, Neil D. and Thomson,Fergus J., Influence of Fixation Conditions on the Performance ofGlutaraldehyde-Treated Porcine Aortic Valves: Towards A More ScientificBasis, Thorax, Volume 34, 1979, describe problems that occur in thestructure of the valve leaflets during tanning under pressures at thephysiological levels. Briefly, the valve tissue of a porcine aorticvalve includes collagen fibers arranged in a waveform or crimpedcondition. When internal pressures are applied to the valve during thetanning process, the collagen waveform tends to straighten out, and thedegree to which the waveform returns to its original condition followingremoval of the internal pressure is a function of the magnitude of theinternal pressure applied during the tanning process. If the collagendoes not return to its original waveform condition, the stiffness of thevalve leaflet is adversely affected. This has an adverse influence onthe manner in which the valve leaflets open, and it provides regions oflocal strain or kinks in the valve leaflets during opening. It isbelieved that loss of the waveform geometry also reduces the fatiguedurability of the valve leaflets.

According to these publications, obtaining fixation at the higherinternal pressure levels results in considerable permanent loss of thecollagen waveform so that, upon removal of the relatively high internalpressure, much of the collagen does not return to its original waveformcondition. It is further reported by the same authors that a pressure of100 mm Hg eliminates the waveform geometry entirely and that pressureseven as low as 4 mm Hg result in significant reductions in the waveformafter removal of the internal pressure.

To avoid these undesirable results, it is suggested in the above-notedpublications that the tanning process should be carried out at pressuresin the range of slightly above zero to about 4 mm Hg. However, aninherent effect of subjecting the valve to the tanning fluid is that thevalve tends to shrink and distort. The relatively low internal pressureof just above zero to about 4 mm Hg is insufficient to retain the animalheart valve against shrinkage and distortion during the tanning process.Accordingly, the valve may become fixed in a configuration which isunsuited for use in an implant.

One prior art method for tanning at essentially zero pressuredifferential across the valve leaflet is to pack the valve leaflet witha batting, such as a Dacron or cotton batting, to assure that theleaflets contact each other to provide a competent valve. However, greatcare must be taken in packing the delicate valve leaflets, and even withthe exercise of considerable care and diligence in packing, themechanical stresses exerted on the valve leaflets cannot be measured andare subject to operator variability. In addition, abrasion between thevalve leaflets and the batting results in changes in surface morphology,and a substantial amount of time is required to carefully pack the valveleaflets with the batting.

SUMMARY OF THE INVENTION

This invention accomplishes fixation of an animal heart valve bysubjecting the valve to a tanning fluid at relatively low pressures toavoid permanent loss of the collagen waveform. During the tanningprocess, the valve is mechanically restrained to substantially avoidshrinkage and distortion, and packing the valve leaflets is eliminated.

With this invention, an animal aortic heart valve is subjected to atanning fluid at a differential pressure across the valve leaflets in adirection to urge the valve leaflets closed. The differential pressureshould be sufficient to hold the valve leaflets closed, and, therefore,should be greater than zero. On the other hand, the differentialpressure should not be so great as to provide a significant percentageof irreversible alterations in the waveform geometry of the collagen ofthe valve leaflets. For a porcine aortic valve and based uponexperimental data presently available, this establishes an upperdifferential pressure limit of about 4 mm Hg, although particularcircumstances and/or additional experimental data may prove that 4 mm Hgis not an absolute upper differential pressure limit.

Because the internal pressure on the valve is insufficient to restrainthe valve against distortion during the tanning process, this inventionprovides for other forms of restraints. Preferably, restraint isaccomplished mechanically by inserting an insert into the aortic valveso that the insert is present for restraining purposes during thetanning process. The insert has numerous features which, whenappropriately employed, provide a particularly advantageous restraintagainst the distortion which inherently tends to occur as a result ofthe tanning process.

To illustrate, an aortic valve may be considered as including a tubularaorta segment and three delicate valve leaflets, each of which isaffixed to the aorta segment at a juncture. The insert preferably has aleading end and is inserted into the aorta segment to a location atwhich the leading end is closely adjacent the junctures. When the valveis subjected to a tanning fluid, it tends to shrink and this tends todraw the valve leaflets inwardly. By inserting the insert to thislocation, the insert appropriately engages the valve to substantiallyprevent it from shrinking and distorting. The insert is preferablyrelatively rigid and it must have sufficient rigidity to enable it toretain the valve against distortion.

It is desirable to avoid contacting the delicate valve leaflets with theinsert, and accordingly, the insert preferably engages the valve on theside of the junctures remote from the valve leaflets. Also, the insertmay advantageously include three axially extending projections separatedcircumferentially by recesses. In the installed condition, the threeprojections engage and restrain the valve adjacent the junctures,respectively, and the recesses confront the commissures to minimizeshrinkage and distortion of the aortic tissue at the valve annulus andin the area of the sinuses of Valsalva

The aortic valve is not symmetrical. More specifically, theinterventricular wall extends to the base of the right coronary valveleaflet making a kind of support for this valve leaflet. The muscularpart of this leaflet can be weakened by trimming, but in any event, theright coronary valve leaflet provides a slight asymmetry. To accommodatethis, one of the projections has an outer tissue supporting surfaceadjacent its free end which lies more radially outwardly than thecorresponding outer tissue supporting surfaces of the other twoprojections. This projection, i.e., the one with an outer surface whichlies more radially outwardly than the others, is located in the sinus ofValsalva of the right coronary valve leaflet for contact with the wallof the aorta segment. This asymmetry of the insert and its orientationare very important in minimizing shrinkage and distortion near the rightcoronary valve leaflet. The asymmetry of the insert can be brought aboutin different ways, such as by different angles of radial outwardinclination on the projections, providing segments near the free ends ofeach of the projections which project radially outwardly differentdistances, etc.

To avoid damaging the valve, the edges of the insert which contact thetissue are rounded in elevation and in axial cross section. The insertis preferably tubular to enable a tanning fluid to be supplied to theinterior of the valve through the insert. The insert can be constructedof any biocompatible material which is inert to the tanning fluid beingutilized and is preferably transparent to facilitate visual placement ofthe insert in the valve.

The invention, together with additional features and advantages thereof,may best be understood by reference to the following description takenin connection with the accompanying illustrative drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a somewhat schematic sectional view illustrating an apparatusfor tanning animal aortic valves in accordance with the teachings ofthis invention.

FIG. 2 is a sectional view on line 2--2 of FIG. 4 of one form of insertconstructed in accordance with the teachings of this invention insertedinto a porcine aortic valve and clamped in position.

FIG. 3 is a perspective view of the insert.

FIG. 4 is a top plan view of the aortic valve of FIG. 2.

FIG. 5 is a simplified view of the insert and valve of FIG. 2 laid flatto show the zone of contact between the two.

FIG. 6 is a plot of differential pressure across the valve leaflets andcollagen strain.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an apparatus 11 for obtaining low-pressure fixation of aporcine aortic valve 13 utilizing a tanning solution 15 and an insert17. With reference to FIGS. 2 and 4, the aortic valve 13 has beenexcised from the heart and it includes a tubular aorta segment which, inthe form illustrated, is a portion of the ascending aorta 19 and threevalve leaflets 21, 23 and 25. The bulk of the myocardial tissue has beentrimmed from the valve 13 to minimize distortion during fixation, andthe coronary arteries (not shown) are tied off. Adjacent edges of thevalve leaflets 21, 23 and 25 meet to form commissures 27 at thejunctions between adjacent valve leaflets. Each of the valve leaflets21, 23 and 25 may be considered as joined to the ascending aorta 19along a juncture 29. The wall of the ascending aorta 19 adjacent thejunctures 29 forms sinuses 31 of Valsalva. The valve leaflet 21 is theright coronary leaflet.

The insert 17 in the embodiment illustrated is integrally molded from asuitable biocompatible material which is compatible with the tanningsolution 15, such as a suitable polyester or acrylic material. Theinsert 17 is preferably sufficiently transparent so that its positionwithin the aortic valve 13 can be accurately visually observed.

The insert 17 includes a tubular peripheral wall 37 of a generallycylindrical configuration which is open at both ends and which defines agenerally axial cylindrical passage 39 extending through the insert. Theperipheral wall has three generally axially extending projections 41, 43and 45, with each adjacent pair of projections being circumferentiallyspaced by a recess 47. The projection 41 terminates outwardly at itsfree end in a segment or rib 49, and the projections 43 and 45 similarlyterminate at their free ends in ribs 51. As best seen in FIG. 2, theribs 49 and 51 project radially outwardly to provide outer tissuesupporting surfaces 53 and 55, respectively. The recesses 47 preventcontact with, and damage to, the valve leaflets 21, 23 and 25 at thecommissures 27. To avoid damaging the tissue which they contact in thevalve 13, the ribs 49 and 51 and the free ends of the projections 41, 43and 45, in the embodiment illustrated, are curved in axial cross sectionas shown in FIG. 2 and are also curved as viewed in side elevation asshown in FIG. 3.

The recesses 47 are similarly formed by smoothly curved edges on theprojections 41, 43 and 45. The leading end portion, i.e., the upper endportion as viewed in FIGS. 2 and 3, is asymmetrical about an axis 57which is the central axis of the passage 39 and of the insert 17.Specifically, the radial spacing between the outer tissue supportingsurface 53 and the axis 57 is greater than the radial spacing betweenthe axis 57 and the outer tissue supporting surfaces 55. The radialspacing between both of the outer tissue supporting surfaces 55 and theaxis 57 is the same. In the embodiment illustrated, the projectionsextend slightly radially outwardly as they extend toward their freeends.

To carry out the tanning process and obtain fixation of the valve 13,the leading end of the insert 17 is inserted into the open end of theascending aorta 19 to the location shown in FIG. 2. The valve 13 is thensuitably affixed to the insert 17 as by sutures (not shown) or a strap59 which encircles the aorta 19 adjacent its open end and clamps it tothe insert. The strap 59 holds the valve 13 against axial movementrelative to the insert 17 and should not be placed on any part of thevalve which will be used in the implant made from the valve 13.

When in position within the valve 13, the projections 41, 43 and 45engage, or lie closely adjacent, the junctures 29, respectively, and therecesses 47 confront, and lie radially inwardly of, the commissures 27,respectively, as shown in FIG. 2. The insert 17 does not contact thevalve leaflets 21, 23 and 25. The projection 41 is provided forsupporting the valve 13 in the region of the right coronary so that theouter tissue supporting surface 53 of the rib 49 engages the aortic wallof the aorta 19 adjacent the right coronary valve leaflet 21. The ribs49 and 51 engage and support the aortic wall of the aorta in the sinusesof Valsalva 31. Except for the sinuses of Valsalva, the outside diameterof the insert 17 is approximately equal to the inside diameter of theadjacent regions of the ascending aorta 19. The radial spacing betweenthe axis 57 and the outer tissue supporting surfaces 53 and 55represents the desired position beyond which the adjacent junctures 29should not be allowed to distort or shrink.

FIG. 5 shows how the projections 41, 43 and 45 thoroughly support thevalve 13 without contacting the valve leaflets 21, 23 and 25. As viewedin FIG. 5, the junctures 29 are generally semicircular, and the ribs 49and 51 engage the aorta 19 just below the junctures. The recesses 47 andthe curvature (as seen in FIG. 5) of the projections prevent contact ofthe insert with the valve leaflets.

The apparatus 11 may be used to tan one or more of the valves 13 whichhave an insert 17 within them as shown in FIG. 2. The apparatus 11includes a tank 61, a header 63, a reservoir 65 coupled to the header63, a pump 67, an intake conduit 69 leading from the tanning solution 15within the tank to the intake of the pump, and a discharge conduit 71leading from the discharge of the pump to the header 63. The header 63is fixed within the tank 61 and has a passage 73 coupled to the conduit71 and to the lower end of the reservoir 65. The tanning solution 15fills the tank 61 to a predetermined height, and the reservoir 65 has anopen top 75 which lies a prescribed distance above the elevation of thetanning solution 15 in the tank 61. The difference in elevation betweenthe levels of the tanning solution 15 within the tank 61 and thereservoir 65 represents the differential pressure across the valveleaflets 21, 23 and 25 at which the tanning process will be carried out.This is a static head and flow is required only to make up for leakage,and a slight initial flow is required to close the valve leaflets 21, 23and 25. In the embodiment illustrated, the head represented by thedifference in these two elevations is 2 mm Hg.

The passage 73 in the header 63 also communicates with a riser 77 havinga stopper 79 mounted on it. The upper end of the stopper 79 is receivedwithin the lower end of the insert 17 so that the stopper and insertcooperate to mount the valve 13 vertically within the tank 61.Additional valves 13 may be similarly mounted on the header 63, ifdesired.

The tanning solution 15 may be of any composition suitable for tanningthe porcine aortic valve 13. For example, the tanning solution 15 may bea suitable solution of glutaraldehyde.

With the components in the position of FIG. 1, the pump 67 can beoperated to pump tanning solution 15 from the tank 61 through theconduits 69 and 71, the header 63 and over the top 75 of the reservoir65 as may be required to maintain the desired static head. The flow, ifany, is exceedingly small to avoid subjecting the leaflets 21, 23 and 25to any velocity head. Thus, the interior of the valve 13, including theinterior of the valve leaflets 21, 23 and 25, is subject to a staticpressure increasing to the height of the top 75. Simultaneously, theouter surfaces of the valve 13 are subjected to the tanning solution 15at a static pressure corresponding to the elevation of the tanningsolution in the tank 61. Thus, the differential pressure to which theaortic valve 13, and in particular, the delicate valve leaflets 21, 23and 25 is subjected corresponds to the difference in elevation betweenthe top 75 of the reservoir 65 and the elevation of the tanning solution15 within the tank 61. This assures that the internal pressure withinthe valve 13 will exceed the exterior pressure so that the valveleaflets 21, 23 and 25 will be urged toward the closed position. Also,because the liquid level in the reservoir 65 cannot rise above the top75, the maximum internal pressure is also regulated. By utilizing aknown volume of the tanning solution 15 in containers of known volume,the differential pressure across the valve leaflets 21, 23 and 25 can bemaintained at the desired nominal value.

The action of the tanning solution 15 on the valve 13 tends to shrinkand distort the valve. However, the engagement of the valve 13 in theregion of the sinuses of Valsalva 31 against the relatively rigidprojections 41, 43 and 45 prevents significant distortion of thesecritical portions of the valve during the tanning process. The tissueadjacent the right coronary valve leaflet 21 contains additional muscleas shown in FIG. 2. The specially configured projection 41 engages theaortic wall adjacent the right coronary leaflet 21 to preventsignificant distortion and shrinkage in this region. Also, the lowerregions of the valve 13 engage the outer periphery of the insert 17 atand below the projections 41, 43 and 45 to also hold these regionsagainst shrinkage and distortion. The valve 13 is subjected to theaction of the tanning solution 15 for a sufficient time to obtainfixation of the valve, and this may take, for example, about eighthours.

FIG. 6 shows the relationship between valve leaflet deformation as afunction of differential pressure across the valve leaflet during thetanning process. As the differential pressure rises from zero to about 4mm Hg, the valve leaflets deform linearly along a segment 101 of thecurve. During this time, deformation is relatively substantial due toextension of the waveform of the collagen or uncrimping of the collagen.Generally, the deformation represented by the segment 101 is not harmfulbecause, when the differential pressure is removed, the collagen returnsto its original waveform or crimped condition. However, above about 4 mmHg of differential pressure, the rate of deformation is substantiallyreduced, and this is graphically indicated by a segment 103 of the curvewhich has a much steeper slope; thus, indicating a much slower rate ofchange of deformation with respect to differential pressure. Thedeformation of the valve leaflets represented by the segment 103 of thecurve results in permanently removing, to a degree that increases withpressure, the collagen waveform rather than elastically stretching oruncrimping of the collagen. This destruction of the collagen waveformproduces the harmful affects described above. Available experimentaldata indicates that the separation of the segments 101 and 103 is atabout 4 mm Hg differential pressure, although some modest variation inthis could reasonably be expected in that tissue is not homogeneous, andthus all heart valves upon which the process is performed would not beexpected to have identical structural properties. Also, as newexperimental data becomes available, refinements of the upper limit onthe differential pressure at which collagen straining becomesunacceptable may become known. Of course, some straining of the collagenmay occur at under 4 mm Hg but generally it is believed that collagenstrain at differential pressures under 4 mm Hg is not particularlyharmful.

Although an exemplary embodiment of the invention has been shown anddescribed, many changes, modifications and substitutions may be made byone having ordinary skill in the art without necessarily departing fromthe spirit and scope of this invention.

I claim:
 1. An insert for use in restraining an animal heart valveagainst distortion during fixation, said insert comprising:a tubularperipheral wall open at both ends and defining a generally axialpassage; said peripheral wall having three generally axially extendingprojections, each of said projections terminating axially outwardly in acurved end surface as viewed in side elevation, each adjacent pair ofsaid projections being circumferentially spaced by a recess; each ofsaid projections having an outer tissue supporting surface closelyadjacent its free end, the outer tissue supporting surface of one ofsaid projections lying more radially outwardly of the axis of saidpassage than the outer tissue supporting surfaces of the other twoprojections whereby said outer tissue supporting surfaces are arrangedeccentrically about the axis of said passage; and at least the exposedsurface of said insert being constructed of a biocompatible material. 2.An insert as defined in claim 1 wherein each of said curved end surfacesis curved as viewed in axial cross section and said outer tissuesupporting surfaces lie farther radially outwardly than the peripheralwall adjacent the projections.
 3. An insert as defined in claim 1wherein each of said projections terminates in an end portion whichincludes a segment projecting radially outwardly, said outer tissuesupporting surfaces being on said segments, respectively.
 4. An insertas defined in claim 1 wherein the tubular wall is transparent.
 5. Aninsert as defined in claim 3 wherein each of said segments includes acurved rib.